Color and Methods for Color Removal
NELSON L. NEMEROW
Associate Professor of Civil Engineering
North Carolina State College
Raleigh, North Carolina
This paper is intended to provide the reader with a clearer insight
of the concept of color. That color is a vital characteristic of waste
waters goes without further mention. One only needs to be aware of
the complaints that arise from colored streams containing little BOD
and considerable DO. The removal of color from waste waters is often
more important as far as the public is concerned than the removal of
BOD. Heretofore it has been given little consideration. Three necessary
steps that must precede actual experimentation of methods of color removal are: (1) color measurement, (2) past experiences with colored
wastes, and (3) color classification.
COLOR MEASUREMENT
There are several instruments available for measuring the intensity
and quality of colored solutions. Some of these are the spectrophotometer,
the filterphotometer, and the color comparator such as that made by
Hellige. All of these instruments measure with varying degrees of
accuracy the quality and/or intensity of color. The measurement is made
with transmitted light. There is no available instrument capable of
measuring reflected color of a water sample. Transmitted light is carried
by an intercepting color selective medium, and the quality is not affected
by turbidity. Reflected light, on the other hand, is light reflected into
the original medium and altered by an intervening transmitting medium,
the quality of which is affected by the reflecting particles.
The kind and accuracy of color measurement depends on the anticipated use of the water. Color measurement in a water purification plant
should be made with a high precision instrument using transmitted light.
Transmitted light is used since it is this light that is observed by the
water purchaser in the different uses of water.
Both transmitted light and reflected light may be used to measure
color in a sewage and waste treatment plant. The measurement of
transmitted light is useful in determining the efficiency of color removal
in the treatment plant. The measurement of reflected light should be
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